Sound disk record and method and apparatus for making same



Jan. 30, 1940. WAN v 2,188,337

SOUND DISK RECORD AND METHOD AND APPARATUS FOR MAKING SAME Filed Dec. 29, 1937 2 Sheets-Sheet 1 FIG. I

AMP 22 FIG.8. 30

INVENTOR. LEON DEWAN A TTORNEK Jan. 30, 1940. D WAN 2,188,337

SOUND DISK RECORD AND METHOD AND APPARATUS FOR MAKING SAME Filed Dec. 29, 1937 I 2 sheets-Sheet 2 "vii I ml 57&

INVENTOR.

LEON DEWAN ATTORNEY.

Patented Jan. 30, 1940 PATENT OFFICE SOIlND DISK RECORD AND METHOD AND APPARATUS FOR MAKING SAME Leon Dewan, New York, N. Y., assignor of fifty I per cent to George H. Callagham'New York,

' Application December 29, 1937, Serial No. 182,221 7 3 Claims. (01. 179-10114) This invention relates to improvements in recording and. reproducing sound and particularly to stylus recording though some of its features are applicable to other branches of thesound art. 1 One object ofthis invention is the provision of a sound system in which the recording is practically free of distortion, due to inertia, resonance, surface noise, etc., andin which. the recording or reproducing stylus cannot leave the phonic groove or overshoot the sound waves when large sound v'olume'occurs.

, Another object is to enable production of very large capacity records, inwhich the grooves may be offine pitch and the record to move with 16 comparatively slow speed past the stylus and still maintain quality.

Another object is to increase the Volume range of 'a record so that sounds of extreme faintness' or loudness can be satisfactorily recorded and reproduced in their correct proportion. I

' According'to this invention, which isapplicable to both vertical and lateral recording on either cylinder or disk, sound currents derived from a suitable .microphone are caused to traverse a light conductive ribbon mounted under tension in a magnetic field in such manner that its movements, resulting from the sound currents cause a small stylus attached to the ribbon to cut a semblance of the original sound waves into a surface of a plastic record that moves past the .stylus., At the same. time a current whose strength varies in accordance with the general volume of the incoming sound is applied in such manner to the conductive ribbon or to a solenoid which controls the height of the ribbon and mounted, that the natural frequency of vibration 'of the ribbon in the direction ofthe cutting operation is at the high audible frequencies in the region'of 8000 cycles, so that little damping is necessary and the higher overtone frequencies 5 are favored in recording.

' The sound recording is monitored by means of a small mirror oscillably mounted and connected tothe recording stylus ,or ribbon in such a mann'er'as tolbevibrated therefrom and reflect a 50 varying amount of light to a photocell whose output actuates the monitors.

' In reproducing, a stylus which follows the sound groove and" is connected to a mirror mounted at the center of a taut ribbon, causes the mirror to oscillate, with the ribbon as the axis, in accordance with the plastic sound waves and reflect a varying amount of light upon a photocell whose output actuates a loudspeaker. By means of a volume component current derived from the reproducer photocell output and applied 6 to the. means connected with the reproducing stylus, the stylus is caused to press against. the record with. a strength varying in accordance to and with the sound volume. The tension and the dimensions of the ribbon mounting and 10 weight of the stylus and mirror are such that the natural frequency of the mounting is in the region of the higher frequencies, say 10,000, so thatthe stylus accuratelyfollows the sound wave outlines.

In the accompanying drawings: Figure 1- isa view in section and plan of one apparatus for recordinghill-and-dale recordings. Figure 2 is a perspective view of the stylus carrying, head thereof.

Figure 3. is adiagram of one electrical circuit used with theapparatus as shown in Fig. 1.

Figure 4 is an enlarged'view illustrating the stylus, actionupon a plastic record made-by the apparatus in Fig. 1. l 25 Figure 5 is a detail view of the cutting end of the" stylus.

Figure 6 is a detail view of the connection of the stylus and its carrying ribbon.

Figure 7 is, a view thereof showing its mirror 30 mounting.

Figure 8 is another electrical circuit used upon this recording apparatus.

Figure 9 is a cross sectional view through the casing and carrier mechanism of the present re- 35 producing head. Figure 10 is a top plane view of the stylus carrier thereof.

Figure 11 is a top plane view of the optical system; 40

Figure 12.is an enlarged view of the stylus and carrier.

Figure 13 is a diagram of the optical system employing a convex mirror. J

Figure 14 is a cross section on a large scale of the record and stylus guide. i Figure 15is an enlarged detail view of the stylus mount.

Figure 16 is a View in side elevation of the reproducer stylus mount and an electric magnetic control therefor to vary the pressure of the stylus P upon the record.

Figure 17 is an end view partly in section of the stylus, its mountand the record, as viewed in the direction'of the arrow in Fig. 16.

Figure 18 is a view of a modified form of reproducer.

Figure 19 is a diagram of the optical system used therewith, showing the stylus and'support in end elevation.

Figures .20 and .21 are top and side elevations illustrating a modified form of the invention for producing the record engaging pressure of the stylus.

Referring to the recording apparatus, and particularly to Figs. 1 and 2, a ribbon I preferably of duralumin is stretched in the mounting 2 and carries a light cross piece 3 to which is cemented a small cutting stylus 4. A field magnet (not shown) is attached to the cutting head support, and has pole pieces which are close to the ribbon and thus cause it to move in a vertical plane when currents varied by electrical impulses, which in turn are varied by and in accordance with sound impinging upon a microphone. The disk record 5, which may or may not have preformed grooves, travels beneath the stylus in the direction of the arrow and receives an impression of the sound waves. A: light slender pin 6 preferably hollow and of duralumin is sharpened at both ends and is held in place with rubber cement to the cross piece 3 and the mounting 2, and serves the purpose of bracing the cutting point of .the stylus against the drag thereon due to the moving record; and to prevent the ribbon I from being twisted thereby and at the same time allowing the ribbon to vibrate vertically freely. Similar pins I and 8 prevent torsional vibration of the ribbon. at its upper portion due to the action of the record on the fulcrum-like point of the pin 6. Where necessary a fourth pin may be used and placed opposite the pin 6 but to one side out of interference with the stylus.

A small mirror 9, for example, .05 by .10, or less in size, is mounted on a very short taut wire ID to be caused to oscillate with the wire as its axis by means of a short light cross piece I I which rests lightly on the cross piece 3 of the ribbon I, .or upon a small wad I I a of resilient material such as felt, it being rubber cemented in place. This mirror reflects a beam of light from the lamp I2 and lens system I3 to form a light patch of rectangular or other form which normally rests partly upon the mask I4 and impinges upon the monitor photocell I5, the vibration of the mirror 9 causing a varying amount of light to energize the photocell and actuate the monitor phones I6.

When the support I8, whose height above the record is determined by an advance ball and screw, is about to be adjusted and the stylus has not yet reached the record, the light patch falls wholly upon the mask but very near its edge, so that as soon as the stylus is lowered sufficiently to touch the record surface, light falls upon the cell and a meter N indicates that the stylus has reached the record surface. As the support I8 or the recording head is further lowered, the reading of the meter I'I increases since the stylus is further pressed into the record against the tension of the strip I causing a further displacement of the stylus position relative to the recording head and a corresponding twisting of the mirror which reveals to what depth the stylus enters the surface of the record. 7

The cutting head mounting 2 is pivoted to the support I8 at I9 and the vertical member thereof has an armature 20 connected thereto and acted upon by a solenoid 2|, which receives a volume component current preferably derived from the current of the microphone in a manner to be described later on. The screws 22 together with the wad 23 of rubber or felt keep the mounting in adjustment.

During the silent periods, the stylus 4, as shown in Fig. 4, travels at an even depth as indicated at 24. The dotted line 24'A indicates the groove surface before cutting. With the occurrence of sound, not only does the stylus vibrate vertically in accordance With the sound waves by virtue of the attachment to the ribbon I, but it is also lowered or pressed down in accordance with the volume of the sound, due to the action of the solenoid2 I, which .carries volume component current. The result of this is that with increased volume, the increased downward pressure upon the stylus and depth of the cut prevents the stylus from jumping or missing the peaks of the sound waves and permits sound waves of a large volume range to be accurately and fully record- 0 ed. Furthermore this feature allows the downward pressure of the stylus to be very light during periods of silence or faint sounds, since pressure for the large sound volume is provided for, and therefore the fainter sounds may be recorded unimpeded by pressure or resistance of the plastic record.

Instead of moving the mounting 2 to produce the above result, the ribbon I itself may receive both a sound current and a volume component current, as illustrated in Fig. 3, wherein direct volume component current is derived from the output of the microphone 25 by the rectifier and filter 26 in a manner well known in the art, this current passing through the ribbon I and prevented from affecting the secondary of the transformer 21 by the by-pass condenser 28. The sound currents are also passed to the ribbon I, but are first passed through a delay circuit 29, so that the sound current waves in the ribbon are slightly delayed as compared with the sound waves in the microphone. The reason for this is that the volume component current variations usually occur somewhat behind the corresponding changes in sound volume with the result that very sudden volume changes are not properly attended by the volume component feature. However, in this case the delay circuit well known in the art, delays the sound waves to such an extent, that the volume component current, derived from an undelayed portion of the microphone output exactly synchronizes with the sound volume changes. Thus the ribbon I executes both vibrational and depth variation motions, and in this case the mounting 2 would be rigidly secured to the support I8 and the mask I4 may be transferred to the other side of the photocell opening so that the light beam reflected by the mirror 9 shall still increase the amount of light passing to the photocell when the sound volume increases.

The ribbon I and the stylus 4 are so light and the tension of the ribbon is such that the natural vibration frequency of the ribbon in the vertical plane is of the order of 6000 to 10,000 cycles per second. The ribbon may be of duralumin and can be about .001 to .003" thick and .006" to 0.18" broad, in order to resist the tension necessary to attain this high frequency in spite of the added weight of the cutting stylus, which should be as light as possible to permit this high frequency to be reached without the necessity of tensioning or shortening the ribbon to such an extent that the amplitude of the vibration is reduced to insufiicient proportions. The dimensions of the ribbon depend upon the weight of the particular stylus used, and may vary accordingly from those given above. "The lighter the stylus, the less the cross sectional area of the ribbon.

This feature not only permits the soundto be recorded practically without resonance frequencies and undesired harmonics with very little damping of the stylus required, but enables the semblance of the-sound waves to be engraved in the plastic medium in, almost perfect conformity to the electrical sound waves due to the lightness of the system and at the same time to the high tension of the ribbon which causes it to vibrate virtually independently of the variable drag of the plastic medium.

In order to have the stylus extremely light, the sound grooves produced thereby'may be very narrow and the stylus may be about .002" thickness and a breadth greater than this in order to resist the recording dragf See Fig. 5.

In place of the ribbon l, two narrow ribbons 30 may be used, as shown in Fig. 6, thus ofierin a broad base support to the cross piece 3' orthe stylus. If the ribbons 30 are not connected together, then one of the ribbons can carry the sound currents and the other the volume component current desired.

If desired the volume component current may be applied to a conductive ribbon associated with the monitor mirror, and a field magnet may be applied thereto so that volume regulation may be accomplished thereby as illustrated in Fig. 7, wherein a conductive ribbon 3| seen in cross section is attached to the opposite end of the cross piece H" to which'the monitor mirror is cemented. The ribbon by moving vertically due to. the currents therein and the presence of a field magnet pole 32 actuates the cross piece I I with the pivot wire or ribbon H! as an axis.

Other, corrective currents besides the volume component current may be applied to any of the conductive ribbons controlling the stylus to improve the quality of the recorded sound under various conditions. For example, where it is desired to favor or accentuate a certain frequency or frequency band, as for example, frequency at which reproducible volume drops due to the finite dimensions of the recording or reproducing stylus, the arrangement illustrated in Fig. 8 may; be used, wherein a portion of the output of the monitor photocell I5 is fed back to any of; the conductive ribbons controlling the stylus 4 through a circuit 33 of a type well known in the art that accentuates the high frequency component desired. Since the movement of the stylus'perse produces the monitor current, the result is a sort of regenerative or feed back effect whereby the vibrations of the stylus, especially in the high frequency range, are amplified.

If desired, the mounting of the monitor mirror may be such that the mirror has a natural oscile lation frequency of 8000 to 10,000 cycles due to the tension of the ribbon 3|. for example. In this case, the resilient material I! A may be rather thick so that the monitor mirror may have a certain amount of free play and favor or exaggerate the frequency components of the sound current in the ribbon 3| to which the monitor ribbon is tuned. In reproducing a record according to the invention (Figs. 9, 10, 11 a small oblong mirror 34 about .09 by .14" in size, though this figure may vary where desired, is mounted upon a short light wire or ribbon 35, which is under tension. A short light needle 36 which may-baa tungsten wire of fineigaugeis attached tothe stiff duralu- 'min cross piece 31 by having its tapered end forcedinto' a hole in the vcross piece. As the hill-and-dale record 38-travels beneaththeneedle 36 in the direction of the arrow the mirror is caused to oscillate in accordancewith the sound or phonic groove with the wire 35 as the axis.

A beam from alight source. .Iconsisting ofx-a spiral or. relativelythick short :filamentx39. is concentrated bythe lens 40 and the cylindrical concave mirror 4| to fall upon a mirror 34,which reflects it concentrated to be imaged upon the mask 42, wherein is .formed a rather. large rec.- tangular opening 43 through which avarying amount of light passes to'the photo cell 44, as the light beam is vibrated in accordance with the recorded'sound waves. 1 I

The lens 40 forms an image of the filament 39 on the mask 42 in the vertical plane, while in the lateral plane the cylindrical mirror 4Ifforms an image in the mirror and the second cylindrical mirror 45' forms an image in the lateral plane on the mask.- The light lines proceeding from a central point on the filament 39 are shown solid, while the dotted light lines represent th boundaries of .the beam.

The optical system is such that while the whole surface of the small mirror is utilized and the light patch on the mask 42 is of large-area and quite intense (being focused. to an image in both lateral and longitudinal planes), the amplitude of the vibration of the light beam'in accordance with the oscillations of the mirror 34 is unaffected by the optical system, since both mirrors 4| and 45 are cylindrical, and is therefore sufiicientl great to affect the'photo cell:

Fig.13 illustrates a modification of the optical system which differs from that of Figs. 9 and 10, in that the small mirror 46 iscylindrical convex instead of plane as the mirror 34. This permits the beam incident thereon to be more convergent and consequently permits the mirror 46 to collect more light in the longitudinal plane without affecting the characteristics of the beam after reflection because of its cylindrical divergence which neutralizes the effect of the increased convergence. l The distance from'the centerof the pivot wire 35 to the tip of theneedle 36 is so smallit may be of the order to A; ineh--that faint recorded sounds or'waves of very low amplitude will cause the mirror 34 to pass through an appreciable angle of oscillation and thereby allow the photo cell 44 to be actuated sufiiciently for sound reproduction. Although'the amplitude ofthe faint recorded sound waves and the consequent vibrationof the light beam is extremely minute, the

fact that-the lightpatch on the mask 42 is quite broad, for example from-V4 to'l", causes an appreciable amount of light surface upon the aperture or photo cell 'to be varied and'produce the required result.

The mirror 34 'as shown is mounted to oscillate on the pivot wire 35 while two metallic ribbons 41 are rather distant from the pivot wire andv serve by their tension to impart'a natural fre-' quency of vibration to the mirror in its oscillations, the tension ofthe ribbons 41 being such -scending thedeclivity of a steep wavefront as The reason for this the record moves by, the needle, due to the fact that its natural frequency is higher-than that of the recorded sound wave, will descend and maintain contact with this wave front surface in spite of the speed of vertical location change it presents. Thus the needle will not jump or skip the hollows of the high frequency sound waves. At the same time the mounting of the needle system is so exceedingly light that sufficient resiliency to follow the full sound wave amplitude and cause the oscillation of the mirror is present in the needle mounting in spite of the high natural frequency thereof. The pressure of the needle upon the record may thus be made extremely light and still enable the full quality of sound reproduction. This pressure is produced against the tension of the ribbons 41 and is adjusted by means of regulating the height of the mirror mounting or support 48 above the record, which in this case is performed by means of the screw 49. The support 48 which is hinged or pivoted to the main tone arm casing50 rests on the shoe shown in detail in Fig. 14. This shoe may have grooves in its underside to engage the grooves of the record in cases where the record grooves are'pre-forme'd, and the peaks of the grooves are suificiently shallow to clear the sound waves tops by a large margin. The shoe is also adjustable laterally by means of the screw 52 so as to center the needle in a groove.

If the shoe 5| guides only the support 48 then this support is hinged at 53 to be adjusted vertically, as shown, to the tone arm casing and also allowed a little free play in the horizontal plane, while the main tone arm is driven to traverse the record by gearing from the turn table (not shown). In the case where the bottom of the shoe 5| is not provided with grooves for lateral guidance, its bottom may be rounded and the support 48 may be counter-balanced to reduce the friction of the shoe on the record. In this case the needle would be caused to find the groove and guide therethrough the support 48 laterally.

In cases where the needle is suificiently sturdy or where the shoe 5| is grooved, the tone arm 50 itself may be well counter-balanced and rest upon the shoe such as 5| to be guided thereby. The cross piece 31 is very light and may be formed of sheet duralumin in the neighborhood of .002 or .003 in thickness and may be rolled around at its end to form a fine hole into which the wire needle 36 is forced. The breadth and height of the cross piece may be about .03 to .08" to transmit the sound waves and support the mirror while along its edges it may have horizontal fins formed by right angle bends to give a reinforced or T beam structure. The needle, which may be of tungsten or other hard metal, is very short in order that it may keep the distance between its tip and the center pivot wire as small as possible, and may be a diameter of about .001" if desired.

From the reproducing amplifier of the photocell 44, a volume component current can be derived in a well known manner by means of a rectifier as described in connection with the recording apparatus, and this current may be caused to traverse one or both of the conductive ribbons 41, while a field magnet (not shown) may have its poles so placed relatively to the ribbons 41 that they will tend to move vertically when the currents. pass therethrough. The direction of the volume component current is such that although during periods of silence, the needle is allowed to rest very lightly upon the record so as to not create much wear, the occurrence of sound causes an increase of downward pressure of the needle on the record in accordance with the volume orthe amplitude of the sound, so that, with very loud sounds, the pressure will be such as to prevent the needle jiunping the record or the sound wave peaks. The same result may be secured otherwise than by feeding the volume component current to the ribbons 41. A modified arrangement is illustrated in Figs. 20 and 21 where the support 48 is mounted upon pivots 53 attached to the tone arm casing 5ll and allowing the support to be swung rotatably with the wire t5 as an axis, and using a solenoid 2% similar to solenoid 2| of Fig. l to act on a similar armature and cause the support 48 to turn and the needle 36 to press down in accordance with the current in the solenoid.

Where an ordinary hill-and-dale record, lacking the features described in Fig. 4, is reproduced, the mean vertical position of the light patch on the mask 42 would remain constant regardless of the sound vibrations. However, with the use of the record having this feature, the mean position of the light patch would shift vertically in accordance with the sound volume and thereby vary the average energization of the photocell 44. If the light patch rests partly upon the lower part of the aperture and the mask 42, then the average light on the photocell would decrease with increased volume. The reverse efiect occurs when the light patch normally falls on the upper part of the aperture and mask. Either arrangement may be used though it is somewhat preferable to have the light patch normally rest on the lower part of the aperture so that the average amount of light is greatest when the sound volume is low.

On the other hand, this effect may be compensated for, if desired, by feeding a volume component current to a solenoid and causing the solenoid to act upon an armature connected with the cylindrical mirror 45, which may be centrally pivoted in this case so that it is swung in a vertical plane in accordance with sound volume to compensate for the vertical shift of the light patch upon the mask 42, and to keep its mean position constant regardless of sound volume. It is also possible to provide this compensation by moving the mask 42 in the vertical plane by the same means in accordance with the sound volume variation.

A lateral cut record can be produced in accordance with this invention by the method shown in Figs. 16 and 17, where a conductive ribbon 54 of duralumin is stretched between the ends of the support 55. A stylus 56 also of duralumin has a fine sapphire cutting point mounted in the lower end thereof, and this stylus is pivoted to the pin 51, which enters a conical depression in the stylus at an angle, and is secured thereto by rubber cement. The top part of the stylus is somewhat rounded off and rubber cemented to the center of the ribbon 54. The poles of a field magnet 58 are placed close to the ribbon 54 in such manner that when sound currents traverse this ribbon it moves in a lateral or horizontal plane, thereby moving the upper and lower ends of the pivoted needle to cause its cutting point to trace a wavy sound groove in the record 58 as it passes by in the direction of the arrow. The angle of the pin 57 is such that it braces the needle against the dragging force of the record as well as the upward force due to pressure upon the record, and which force would cause the ribbon to sag upwardly. This ribbon is mounted under such tension that the natural period thereof in the horizontal or lateral plane together with the stylus which is extremely light, is that of the upper audible frequencies, for example anywhere between 6000 and 10,000 cycles or over. The ribbon movement may be dampe ened if desired by mechanical means to be applied over the stylus 56. The support 55 may be pivoted at 60 to cutting holder 6!, which may be mounted over the record in a well known manner. The solenoid 62 similar to solenoid 2i, carries volume component current and by acting upon the support 55 through the armature 63 causes the needle pressure on the record to vary with sound volume. The result of this is that when sound of large volume occurs, the needle is pressed further into the record vertically at right angles to the wave form so that not only is the amplitude of the recorded loud sound wave limited but the downward pressure with soft or low amplitude sound waves can thus be made very light to cause the needle to vibrate unimpededly. The contrast between sound waves of large and small amplitudes is thus lessened and the volume range of the record is increased. Further, when such a record is reproduced, the increase in depth with loud sounds tends to prevent thereproducing needle from jumping the groove on account of large volume.

The method for best reproducing such a record is shown in Figs. 18 and 19, wherein the mirror 64, similar to mirror 34, though larger where desired, may be associated with an optical systern similar to that of Fig. 9, and a photocell and amplifier to produce the same effects. This mirror is mounted through a cross piece 65 upon a pivot ribbon 56, which is stretched upon a support 61. A light needle holder 68 which may be less than A; inch, may be formed by means of a tapered hole to hold a wire needle of very fine gauge as before described. A slender pin 69 braces the needle holder against the record drag. The spurs 10 extending from a support 61 press upon the ribbon 66 and preventits upward sagging due to its pressure upon the record. The support Si is pivoted on the reproducer tone arm H which in this case is fixed in its height above the record when traversing the same, so that when the solenoid 12 similar to solenoid 62, and receiving volume component current, acts upon the support 61 through the armature 13,

the needle pressure on the record varies with sound volume so that, for example, when an ordinary lateral cut record is played, the needle cannot jump tonthe next phonic groove due to the greater downward pressure of the needle when loud sounds'occur. In cases where the tone arm is not fixed in its height above the record during traversing the same, it may be made exceedingly ,light and be made integral with the support 61, while the solenoid 12 may act between the tone arm and the tone arm post to vary the vertical pressure of the tone arm upon the record.

Referring back to Fig. 15 there is here shown a method of mounting the mirror 34" without the use of a pivot wire such as 35. The cross piece 31" has extending at right angles thereto projections 31A upon which rest the points of spurs 313 which extend from the mounting support 48. The tension of the ribbons 1! cause HA and 3713 to press against each other and form a pivot.

In the reproducer, such as described in Fig. 9 etc. the current for causing the needle 36 to be pressed down upon the record with a strength depending upon the sound volume, may be derived where the groove itself is of variable depth, from the varying average strength of the photocell current itself as the mean position of the light patch on the mask 42 varies its mean position in accordance with the depth of the groove.

Referring back to Figs. 9 and 14, the shoe 5| need not be stationary but may be in the form or a roller on an axle attached to the mounting so that friction with the record is eliminated. The roller may either have a smooth rounded periphery or may have grooves therein if desired to engage the record grooves in the record when they are pre-formed.

In- View of the fact that this invention calls for a needle bearing down on a record with a variable pressure, rather than a uniform pressure as has hitherto been the practice in the art, it would be desirable to counteract any influence that the variable friction between the needle and the record may have on the speed of the turntable drive. For this purpose the volume component current produced in the device may be caused to control the governor of the phonograph to a slight extent just sufficient to neutralize the efiect of the variable friction as the volume varies. This application is substantially identical with the application Serial No. 10,469 which was held abandoned June 10, 1937.

What is claimed is:

1. In sound reproducing from a phonic groove record, means for converting the vibrations of the record engaging stylus into sound waves and means for causing the stylus to engage the record with a pressure varying in accordance with the sound volume.

2. In a sound apparatus, a stylus actuated pick up for engaging a stylus with a record, electrical means controlled by the pick up and means controlled by said electrical means whereby the volume component of the sound in the record controls the engagement pressure of the stylus with the record.

3. The method of lateral out sound recording which consists of vibrating the stylus laterally according to sound waves and simultaneously varying the engagement pressure of the stylus with the record transversely to the plane of sound vibration in accordance with the sound volume.

LEON DEWAN. 

